1. Field of the Invention
The present invention relates to a surface voltmeter for measuring a surface voltage on a semiconductor substrate.
2. Description of the Background Art
Conventionally, a noncontact-type surface voltmeter has been used for measuring a surface voltage (surface potential) on a photoconductor drum of a copy machine or a substrate having ICs mounted thereon. Japanese Patent Application Laid-Open No. 6-88846 (Document 1) discloses a probe for a DC type surface voltmeter for measuring photo-induced discharge characteristics of a charged photoreceptor or the like. In the probe, an optical fiber coated with a transparent conductive film is used as a detecting electrode and light can be applied to a small measurement area which is close to the detecting electrode at a small distance.
Some types of surface voltmeters utilize an alternative method. In the alternative method, an electrode which vibrates at a predetermined amplitude is positioned close to an object to be measured, and a surface voltage is obtained on the basis of a displacement current from the electrode which is generated by change of a capacitance between the electrode and the object. In Document 1, it is described that the above probe may be also used in measurement of a surface voltage by the alternative method.
In recent years, the surface voltmeter which utilizes the alternative method has been also used in measurement of an amount of charge which is charged on a surface of an insulating film such as an oxide film formed on a semiconductor substrate. However, a surface voltage measured by a normal surface voltmeter is a sum of a voltage generated by the charge which is charged on the surface of the insulating film and a voltage (hereinafter, referred to as “induced voltage (induced potential)”) generated by hole(s) or electron(s) which is induced in the vicinity of a boundary between the main body of the semiconductor substrate and the insulating film, by the charged charge. For precisely measuring only the voltage generated by the charge which is charged on the surface of the insulating film, it is considered that light having an energy over a band gap of a semiconductor is applied to the semiconductor substrate and excessive electron-hole pairs are generated in an irradiation region to make the induced voltage approximately 0, and then a surface voltage is measured.
In measurement of the surface voltage on the semiconductor substrate, since the electrode is positioned very close to the semiconductor substrate for improving the measurement accuracy, it is difficult that a light source is located on a side of the electrode and light is applied to a measurement area from a small space between the electrode and the semiconductor substrate. If the surface voltmeter of Document 1 is applied to measurement of the surface voltage on the semiconductor substrate, though it is possible to apply light to the measurement area, the surface voltmeter is for measuring the photo-induced discharge characteristics of the photoreceptor or the like and it is therefore expected that high measurement accuracy is difficult to acquire. Also, since a plurality of optical fibers and insulation material, guard electrodes and the like which are provided around the plurality of optical fibers should be vibrated as a unit, the load applied on a vibration part in vibrating is increased.
As one of surface voltmeters for performing measurement while applying light to a substrate, a surface voltmeter having a construction where a light source is positioned above a transparent electrode facing a semiconductor substrate and an electromagnetic siren type-vibration part for holding an upper surface of the transparent electrode from its side is provided has been known. In the surface voltmeter, normally, a lower surface of the vibration part (i.e., the lower surface facing the semiconductor substrate) is covered with a cover and an electrode wiring through which a displacement current from the transparent electrode flows extends in the horizontal direction between the vibration part and the cover.
In such a surface voltmeter, however, since noise components caused by a stray capacitance between the cover and the electrode wiring which is vibrated together with the transparent electrode and a stray capacitance between the cover and the vibration part are generated in vibrating the electrode by the vibration part, it is difficult to perform high accurate measurement of a surface voltage required for evaluation of a semiconductor substrate.